1. Field of the Invention
The present invention relates to a current resonance type switching power supply device, more particularly to a switching power supply device which prevents a through current from flowing through switching elements when the switching power supply device is overloaded.
2. Description of the Related Art
FIG. 6A illustrates a circuit configuration of a conventional current resonance type switching power supply device. Referring to FIG. 6A, a full-wave rectifying circuit 2 rectifies alternating-current (AC) voltage from a commercial AC power supply 1 and outputs the voltage rectified by the full-wave rectifying circuit 2 to a smoothing capacitor 3. The smoothing capacitor 3 smoothens the full-wave rectified voltage output from the full-wave rectifying circuit 2, and outputs a direct-current (DC) voltage Vdc.
A series circuit constituted of two switching elements, complementary metal-oxide semiconductor field-effect transistors (MOSFETs) 8 and 9 (hereinafter, the MOSFET 8 is referred to as a high side FET 8 and the MOSFET 9 is referred to as a low side FET 9) is connected to both ends of the smoothing capacitor 3.
A transformer 11 is connected in parallel to the low side FET 9. The primary winding of the transformer 11 is expressed equivalently by an excitation inductance 12 and a leakage inductance 13. The leakage inductance 13 and a current resonance capacitor 14 constitute a series resonance circuit.
The leakage inductance 13 may be configured separately from the transformer 11. A voltage resonance capacitor 10 is connected in parallel to the low side FET 9.
The secondary winding of the transformer 11 is formed of two coils each having a different phase, one of which is wound to generate a common mode voltage to the primary winding, and the other one of which is wound to generate a reverse phase voltage to the primary winding. To the secondary winding of the transformer 11, a rectification and smoothing circuit constituted of a diode 15A, a diode 15B, and a smoothing capacitor 16, and a load resistor 17 (which indicates a connected load) are connected.
This rectification and smoothing circuit full-wave rectifies and smoothens a voltage (pulse voltage ON/OFF controlled) induced in the secondary winding of the transformer 11 to output a DC output voltage Vout to the load resistor 17.
The DC output voltage Vout is input to an error amplifier in a shunt regulator 19. The error amplifier compares the Vout with a reference voltage, and supplies an error signal corresponding to the value of the detected error to a photo-coupler 21. The photo-coupler 21 feeds back the error signal from the secondary winding to the primary winding with insulation between the primary and secondary windings maintained. A resistor 20 is a limiting resistor inserted to limit a current supplied to the light emitting diode (LED), which is a light emitting element, in the photo-coupler 21.
A control circuit 7 changes the oscillation frequency of an oscillator (not illustrated) incorporated in the control circuit 7 according to the value of a current flowing through a photo transistor, which is a light-receiving element of the photo-coupler 21. When the oscillation frequency of the oscillator is changed, the switching frequencies of the high side FET 8 and the low side FET 9 change, so that the amount of energy transmitted from the primary winding to the secondary winding also changes.
As a result, the value of the DC output voltage Vout output from the secondary winding is controlled variably. The control circuit 7 provides each gate terminal of the high side FET 8 and the low side FET 9 with a period (dead time) when no voltage is applied thereto to control the voltage, and controls both the FETs 8 and 9 to turn on/off alternately while prohibiting both of them from turning on at the same time.
In such a current resonance type switching power supply device, the control circuit 7 performs control so that, as the DC output voltage Vout from the secondary winding drops, the switching frequency decreases. Consequently, the amount of energy transmitted to the secondary winding is increased. Conversely, when the DC output voltage Vout of the secondary winding rises, the control circuit 7 controls the switching frequency to increase so as to reduce the amount of energy transmitted to the secondary winding.
FIG. 6B illustrates the detail of a resonance circuit section on the primary winding side of the transformer in FIG. 6A. As illustrated in FIG. 6B, the high side FET 8 contains a parasitic diode (or body diode) D1 and the low side FET 9 contains a parasitic diode (or body diode) D2.
FIG. 7A illustrates an operation waveform of the circuit in FIG. 6B. In FIGS. 7A and 7B, symbol VQ1gs indicates a gate signal for driving the high side FET 8, symbol VQ2gs indicates a gate signal for driving the low side FET 9, symbol IQ1 indicates a current flowing through the high side FET 8, symbol IQ2 indicates a current flowing through the low side FET 8, symbol Ires indicates a current flowing through the resonance circuit, and symbol Vcr indicates a voltage between both ends of the current resonance capacitor 14.
First, an operation of the circuit in a state where its input voltage and a load current are stabilized (steady time) will be described with reference to FIG. 7A. In a period A when the high side FET 8 is ON and the low side FET 9 is OFF, a current flows from the high side FET 8 to the leakage inductance 13 to the current resonance capacitor 14.
Energy is stored in the current resonance capacitor 14 via the excitation inductance 12 of the primary winding of the transformer 11 and the leakage inductance 13, and consequently a voltage applied between both ends of the current resonance capacitor 14 rises.
Next, in a dead time period B when both the high side FET 8 and the low side FET 9 are OFF, a current flows from the body diode D2 of the low side FET 9 to the leakage inductance 13 to the current resonance capacitor 14.
A zero-voltage switching (hereinafter referred to as ZVS) is achieved by turning ON the low side FET 9 when a current is flowing through the body diode D2.
Next, in a period C when the high side FET 8 is OFF and the low side FET 9 is ON, charge to the current resonance capacitor 14 is continued, and when discharge of energy stored in the leakage inductance 13 ends, the direction of the resonance current changes, so that a current flows from the current resonance capacitor to the leakage inductance 13 to the low side FET 9. At this time, the voltage of the current resonance capacitor 14 drops.
Next, in a dead time period D (period when both the FETs 8 and 9 are OFF like the period B), a current flows from the current resonance capacitor 14 to the leakage inductance 13 to the body diode D1. The ZVS is achieved by turning ON the high side FET 8 when a current is flowing through the body diode D1.
As described above, at the time of a stable (steady) operation, the leakage inductance 13 and the current resonance capacitor 14 carry out the resonance operation to control the switching frequencies of the FETs 8 and 9 variably. As a result, a voltage supplied to the primary winding of the transformer 11 is changed to control energy transmitted to the secondary winding variably.
Next, an operation under a low input voltage and a large load current after the voltage is changed will be described below. In a conventional current resonance type switching power supply device, the driving frequency of the resonance circuit is much lower than the resonance frequency of the resonance circuit, when a voltage (Vdc) input into the resonance circuit is low and the resistance of the load resistor 17 is small (the load current is large).
At this time, a through current that flows via the body diodes D1 and D2 of the high side FET 8 and the low side FET 9 is generated, thereby damaging the high side FET 8 and the low side FET 9. There is a possibility that the FETs may be destroyed depending on the magnitude of the through current.
Next, the operation waveform when this through current is generated will be described with reference to FIG. 7B. At a point E in FIG. 7B, it is assumed that the high side FET 8 is ON and the low side FET 9 is OFF. At this time, a current flows from the high side FET 8 to the leakage inductance 13 to the current resonance capacitor 14, and consequently, the voltage of the current resonance capacitor 14 rises gradually.
Next, the control circuit 7 turns OFF the high side FET 8 and keeps OFF the low side FET 9 at a point F according to information from the secondary winding fed back by the photo-coupler 21. Because the control circuit 7 drives the FETs 8 and 9 at a frequency lower than the resonance frequency of the resonance circuit, the discharge of the energy stored in the leakage inductance 13 ends in the period F to G, and then, the direction of the resonance current changes.
At this time, a current flows from the current resonance capacitor 14 to the leakage inductance 13 to the body diode D1 of the high side FET 8. After this dead time period (F to G), the low side FET 9 is turned ON at a point G. A current continues to flow from the current resonance capacitor 14 to the leakage inductance 13 to the body diode D1 of the high side FET 8.
Thus, when the low side FET 9 is turned ON at a point G, the through current flows, short-circuiting the power supply Vdc and the ground in a path of the body diode D1 (reverse recovery) to the low side FET 9 in a period until carriers in the body diode D1 are extinguished.
The amount of change (inclination) of this through current per unit time is large. That is, an excessive current flows instantaneously. Consequently, parasitic transistors in the FETs 8 and 9 are turned ON so that an excessive load is applied onto the FETs 8 and 9, and sometimes, the FETs might be destroyed.
Japanese Patent Application Laid-Open No. 2005-198457 discusses a method for solving this problem about the flow of the through current. A current resonance type switching power supply device according to Japanese Patent Application Laid-Open No. 2005-198457 detects a current flowing in the body diode of the switching element (FET), and controls the switching element not to turn ON or OFF in a period when a current flows in the body diode.
As another solution method, according to Japanese Patent Application Laid-Open No. 2007-006614, the switching operation is controlled by detecting a voltage applied between the drain and the source of the FET acting as a switching element to detect the direction of the resonance current, thereby holding the OFF period of the FET.
However, in the switching power supply device according to Japanese Patent Application Laid-Open No. 2005-198457, the switching state of the FET is held in a period when a current is flowing through the body diode. Thus, when the FETs are overloaded, until the flow of a current to the diode is stopped, the high side FET and the low side FET continue to be OFF to reduce the output voltage. In this state, no sufficient output can be obtained when the load condition changes.
In the switching power supply device discussed in Japanese Patent Application Laid-Open No. 2007-006614, after the FET is turned OFF, there occurs a period when the voltage applied between the drain and the source of the FET remains unstable due to ringing after that switching of the FET. As a result, the accuracy of a detected voltage is not high in the period when the voltage is unstable and consequently, the direction of the resonance current sometimes might not be captured accurately.